Working on support for XML Schema 2 Datatypes.

* [DATETIME_META,ISO31,DOC] Added meta-data information on several datetime standards.
* [ISO8601_GENERIC,SC] Added float and time designator (from module ISO8601_TIME).
* [ISO8601_TIME_INTERVAL,EXT] Added iso8601_duration//2 to public predicates, using the duration datatype.
* [STD_META,NEW] New module supporting meta-data assertions about standards.
* [W3C_DATETIME,DOC] Added note on datetime send to W3C.
* [XML_DATATYPES,SC] Added xml_char//1 to public predicates.
* [XML_SCHEMA_DATATYPES,NEW] Added xmls_canonical_value/3, xmls_[boolean,decimal,string]/2.
* [XMLS_DATETIME,NEW] New module supporting the date-time datatypes of XML Schema 2:
** duration_smaller_than/2.
** 'fQuotient'/[2,3]
** modulo/[2,3]
** summate_time/3 (+ unit tests).
** xmls_duration//3 (+ unit tests)